Other Criteria of Open Source Software

GPL Compatibility

Making a software free is not necessarily enough to make it compatible with
the GNU GPL licence. The GPL makes some restrictions regarding which
licences it can link against, and some perfectly good free software does
not qualify to it. (examples are the Mozilla Public License, the Qt Public
License, and even the original BSD licence). It is advisable that whenever
possible a developer or vendor should choose a licence that is compatible with the
GPL, because otherwise there may be problems integrating his code
with GPLed one or using both a GPL and a non-GPL compatible library. (I am
not a lawyer, so I cannot conclusively say when it is legal or not).

Mozilla is an example for a large
project that started out with its custom (albeit now relatively common),
non-GPL compatible licence, and recently adopted a triple licence of the
Mozilla Public License, the GNU General Public License, and the GNU Lesser
General Public License in order to make it compatible with the GPL and to
standardise its integrability. The Qt library whose commercial vendor and
originator is Troll Tech Inc.,
also adopted the GPL as well as its own QPL, to
relieve the various legal problems that
KDE (a desktop system for UNIXes which
is based on it) faced when using GPL code.

Copyleft

Copyleft means that a derived work of a copyleft software, that are not
used for internal or personal use, must include the source code and
released under the same terms of the original work. Copyleft is common in
many licences including the GPL, the Lesser General Public License, the
QPL, etc.

Many licences are not Copyleft - most notably the BSD and MIT/X11 licences.
Software released under such licences can be derived into a proprietary
software product by a third party, and often have been.

Open Source vs. Sourceware

Open source does not mean any software that is accompanied by its source,
albeit many people who are new to the term would be tempted to think that.
It is possible to write non-OS software while accompanying it with the
source.

Examples for such cases are:

1. The Microsoft Visual C++ Run-Time Library and the Microsoft Foundation
Classes, that are accompanied with their source.

2. xv - a very popular
shareware image viewer and manipulator for X-Windows that has been
distributed with its source code.

3. qmail - a popular mail server
whose source code is available and can be deployed free of charge, but its
licence specifies that it is illegal to distribute modified binaries (at least
outside the organization) This is enough to make it non-open-source, but it is
still a very popular program.

None of these packages qualify as free software, but they are all
accompanied with the source. There are many others around. A quick search
on Freshmeat will find many such
packages.

In order for a program to be open-source it needs to be free of various
restrictions as specified in the
open-source definition. To be free software as well,
it must be also free of some other restrictions.
[3]

I believe the term open-source is a bit dangerous in this regard. Then
again, free software may not automatically be associated with freedom and
liberty, so it isn't perfect either. But I guess finding a description that
accurately describes it in a short space is not very possible, so these
terms will have to do.

[3]
In a recent query given to an Israeli Defence Force Official (IDF) by an Israeli
member of the parliament, the former interpreted open-source as software that the IDF has access
to its source. This is an even more radical deviation from the correct
meaning.